CVE-2025-21394 is a use-after-free vulnerability (CWE-416) identified in Microsoft Office Online Server version 1.0.0, specifically impacting the Excel component. Use-after-free vulnerabilities occur when a program continues to use memory after it has been freed, potentially leading to arbitrary code execution, memory corruption, or crashes. In this case, the vulnerability enables remote code execution (RCE) through maliciously crafted Excel content processed by the Office Online Server. The CVSS 3.1 base score of 7.8 reflects a high-severity issue with an attack vector classified as local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). The scope is unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). This means an attacker can execute arbitrary code with the privileges of the Office Online Server process, potentially leading to full system compromise. The vulnerability is currently published with no known exploits in the wild and no patches publicly available yet. The flaw likely arises from improper memory management in Excel Online Server’s handling of user input or document processing, causing use-after-free conditions that can be triggered remotely when a user opens or interacts with malicious Excel content. Given the critical role of Office Online Server in providing browser-based Office functionality, exploitation could allow attackers to bypass traditional endpoint protections and compromise server-side environments.

For European organizations, this vulnerability poses significant risks, especially for enterprises and public sector entities relying on Microsoft Office Online Server for collaborative document editing and Excel processing. Successful exploitation could lead to remote code execution on servers, enabling attackers to steal sensitive data, deploy ransomware, or disrupt business operations. Confidentiality breaches could expose personal data protected under GDPR, leading to regulatory penalties and reputational damage. Integrity and availability impacts could disrupt critical workflows dependent on Office Online Server, affecting productivity and service delivery. Since the attack requires user interaction, phishing or social engineering campaigns could be used to lure victims into triggering the exploit. The local attack vector implies that attackers may need some form of network access or ability to deliver malicious content to users of the service. Organizations with exposed Office Online Server instances or insufficient network segmentation are at higher risk. The absence of known exploits in the wild provides a window for proactive mitigation, but the high severity score demands urgent attention.

1. Monitor Microsoft security advisories closely and apply official patches or updates for Office Online Server as soon as they become available. 2. Restrict network access to Office Online Server instances using firewalls and network segmentation to limit exposure to untrusted networks. 3. Implement strict content filtering and scanning for Excel files uploaded or accessed via Office Online Server to detect and block malicious documents. 4. Educate users about phishing and social engineering risks to reduce the likelihood of triggering the vulnerability through malicious content. 5. Employ application-layer security controls such as Web Application Firewalls (WAFs) configured to detect anomalous requests targeting Office Online Server. 6. Regularly audit and monitor server logs for unusual activity or signs of exploitation attempts. 7. Consider deploying Office Online Server in isolated environments or using virtualized containers to limit the blast radius of potential compromises. 8. Disable or limit Excel Online functionality if not required, reducing the attack surface. 9. Use endpoint protection solutions with behavioral detection capabilities to identify exploitation attempts on client devices interacting with Office Online Server.